The invention relates to the field of intravascular balloons, and more particularly to method and apparatus for forming balloons.
In percutaneous transluminal coronary angioplasty (PTCA) procedures a guiding catheter is advanced until the, distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of an dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient""s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter, having, an inflatable balloon on the distal portion thereof, is advanced into the patient""s coronary anatomy over the previously introduced guidewire until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with liquid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom.
In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method, of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant an intravascular prosthesis, generally called a stent, inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Thus, stents are used to open a stenosed vessel, and strengthen the dilated area by remaining inside the vessel.
In either procedure, substantial, uncontrolled or unpredictable expansion of the balloon against the vessel wall can cause trauma to the vessel wall. For example, although stents have been used effectively for some time, the effectiveness of a stent can be diminished if it is not properly implanted within the vessel. Additionally, the final location of the implanted stent in the body lumen may be beyond the physician""s control where longitudinal growth of the stent deploying balloon causes the stent""s position on the balloon to shift during deployment. As the balloon""s axial length grows during inflation, the stent may shift position along the length of the balloon, and the stent may be implanted upstream or downstream of the desired location in the body lumen. Thus, balloons which have a large amount of longitudinal growth during inflation can frequently provide inadequate control over the location of the implanted stent. Thus, it is important for the balloon to exhibit dimensional stability.
Therefore, what has been needed is an improved method for forming catheter balloons. The present invention satisfies these and other needs.
The present invention is directed to an apparatus and method for forming balloons with improved dimensional stability and balloons formed by the same.
The method of the present invention provides for a very accurate control of the temperature profile of the balloon material during its making. The attributes of the balloon can be affected by how the balloon is treated during the blowing stage and after the initial blowing, i.e., heat-setting. Using the present method, the balloon will form more uniformly and evenly (e.g., wall thickness and outer diameter of the balloon). The present method significantly increases the dimensional stability of the balloon which provides a balloon that is more predictable in use. The present heat-set process also provides the means for the working length to be located more accurately on dilation catheters and stent delivery systems.
In one embodiment, the method for forming the balloon comprises disposing a polymeric tubular product having an effective length with first and second ends within a mold. The interior of the tubular product is then pressurized. At least a portion of the tubular product is heated to a first elevated temperature for a first predetermined period of time to form the tubular product into a balloon. Preferably, the temperature of the tubular product is maintained to a minimal temperature differential from the first temperature. The tubular product is heated to a second elevated temperature for a second predetermined period of time to heat set the formed balloon. The tubular product (i.e. formed balloon) is then cooled down to substantially ambient temperature and may be subsequently removed. In an embodiment, the temperature differential is less than about 100xc2x0 C., preferably, less than about 50xc2x0 C., and more preferably, less than about 20xc2x0 C. In one embodiment, the first elevated temperature is greater than the glass transition temperature of the polymeric material forming the tubular product, preferably, by at least 10xc2x0 C., more preferably, by at least 20xc2x0 C., and most preferably, by at least 40xc2x0 C. Preferably, the first elevated temperature is less than the melting temperature of the polymeric material forming the tubular product. The second elevated temperature may be equal or greater than the first elevated temperature, and is preferably sufficiently high to thermoset the polymeric material forming the tubular product.
In one embodiment, the tubular product is heated uniformly between the first and second ends to the second elevated temperature for a predetermined period of time to heat set the formed balloon. Preferably, the temperature difference between the first and second ends is less than about 30xc2x0 C., more preferably, less than 15xc2x0 C., and most preferably, less than 10xc2x0 C.
In a preferred embodiment, the tubular product is heated to the first elevated temperature with a first heating member, and to the second elevated temperature with a second heating member. The first heating member may apply the heat as it traverses along the length of the mold. Alternatively, the first heating member has an effective length which is at least substantially the same as the effective length of the tubular product. In this embodiment, the first heating member may then apply the heat to the mold simultaneously across the effective length of the tubular product.
In one embodiment, the second heating member applies heat to the tubular product as it traverses from one end of the tubular product to the other end. Alternatively, the second heating member may apply the heat to the tubular product simultaneously across the effective length of the tubular product.
In another embodiment, the first and second heating members are integral with one another. Alternatively, the first heating member and the second heating member may be on different heating heads. The second heating member may apply the heat to the mold as it traverses along the length of the mold or it may apply the heat simultaneously across the effective length of the mold, and thus, the tubular product.
Balloons formed from the process of the present invention, preferably, have either or both a reduced radial shrinkage and reduced axial growth. Such reduction, being in radial shrinkage or axial growth, preferably, is less than about 10%, more preferably, less than about 6%, and most preferably, less than about 4%.